1. Field of Invention
An improved pneumatic vibration isolation system which has additional stability for high center-of-gravity payloads.
2. Description of the Relevant Art
In many sensitive instrumentation applications it is desirable to isolate a payload from ground vibrations. It is well known to integrate a pneumatic air spring with a simple pendulum to isolate from vertical and horizontal ground noise respectively. Air springs have an advantage over conventional springs in that they can maintain a payload at a given operating height independent of changes in the payload's weight. The vibration isolation characteristic of such isolators is also largely independent of the payload's weight. Payloads are generally supported by at least three isolators, with four being the most common number. More isolators can be used to support additional weight, with little change on the isolation system's performance. The height of the payload is maintained in such systems by a mechanical or electronic valving system which monitors the payload's height and adjusts the amount of air in each isolator. In this way the isolators can return to the same height with changing or shifting payload weights. Pneumatic isolators are typically of a two-chamber design, where motion of the payload forces air to move through a small orifice or flow restrictor. The resistance to this flow provides vertical damping in the isolators.
Payloads supported below their center-of-gravity (c.g.) are inherently unstable. As the payload tilts, its center-of-mass moves horizontally in a way that wants to further increase tilt. Opposing this is the vertical stiffness of the pneumatic isolators which try to restore the payload to level. The balance of these two forces determines whether the system is gravitationally stable or not. Stability of a system increases with the square of the isolator separation, and decreases linearly with the payload's center-of-mass height.
Although system tilt stability can be improved by simply increasing the separation between the isolators, many instruments which use pneumatic isolators are forced to have as small a footprint as possible. This is particularly true for instruments designed for the semiconductor manufacturing industry, where ‘cleanroom’ floor space can be extremely costly.
There are two basic configurations for integrating vertical pneumatic isolators with pendulums for horizontal isolation. In one geometry, the pendulum is between the pneumatic isolator and the payload. In these, floor vibration is transmitted through an air spring, then the pendulum. These are called ‘spring-first’ isolators. If the pendulum is between the floor and the air spring, then it is called a ‘pendulum first’ isolator.
The disadvantage of spring-first systems when used to support high c.g. payloads is two fold. First, the effective support point for a spring-first isolator is the bottom of the pendulum. This increases the effective c.g. height by the length of the pendulum wires, decreasing the tilt stability. This type of isolator also produces tilt when the payload is translated in a horizontal direction. As the payload moves sideways, the burden of supporting the payload is shifted from the isolators on one side of the payload to the other. This causes the isolators more heavily burdened to depress, and those with a lightened load to extend. This generates a tilt from a pure horizontal displacement (known as horizontal-to-tilt coupling). This type of coupling also tends to decrease system stability.
In pendulum first isolators, the vertical pneumatic isolators move with the payload when displaced horizontally, and thus the weight burden on the isolators is unchanged, and this contribution to tilt instability is eliminated.